The invention concerns a nuclear magnetic resonance coil probe head comprising at least two coil/resonator configurations A1 and A2, wherein at least one of the coil/resonator configurations A1 has two saddle-shaped coils S1 and S2, wherein each coil has one window which is surrounded by N windings connected in series, wherein N≧2, wherein the coil/resonator configurations A1 and A2 are aligned perpendicularly to each other, and wherein the coil/resonator configurations A1 and A2 have different resonance frequencies.
A probe head of this type is disclosed in U.S. Pat. No. 6,175,237 and the references cited therein.
U.S. Pat. No. 4,739,269 discloses an imaging NMR apparatus which comprises a transmitting coil designed as a volume coil and a receiver coil designed as planar surface coil. In order to prevent input of disturbing signals from outside of the measuring volume, the receiver coil is symmetrized. The receiver coil has two series-connected windings.
U.S. Pat. No. 6,201,392 B1 describes an NMR probe with several pairs of planar coils, at least one of them being an HTS coil pair. The respective first coils of each pair are oriented coplanar to each other, and the respective second coils of each pair are oriented coplanar to each other, wherein coupling of the coil pairs is minimized.
Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful methods of instrumental analysis. Electromagnetic pulses are thereby irradiated into a sample which is exposed to a static magnetic field. The sample emits a characteristic electromagnetic response based on the properties of the nuclei in the sample.
An NMR probe head usually has one or two preferred measuring frequencies which are used for detection. Further measuring frequencies are mainly used for transmitting decoupling pulses. These either eliminate interactions between the nuclei to be observed and the other nuclei in the surroundings thereof, or are used for “inverted experiments” which examine the effect of decoupling nuclei with a low gyromagnetic relationship γ in the spectrum of a nucleus with a high γ (in particular 1H).
Two nested coil configurations are generally used to permit simultaneous operation at several measuring frequencies. At least one of the detection measuring frequencies is thereby placed on the inner coil configuration and one tries to minimize the loss of these coil configurations. This optimizes the sensitivity of the probe head during detection.
A coil configuration typically consists of two coils which are usually connected in parallel or in series using feed lines. Coil configurations which are not galvanically or capacitively coupled to each other or consist of only one coil can also be realized. Complicated coil configurations are formed from a plurality of coils. Each coil may comprise several windings which are enclosing a window, and may be designed as one path or several parallel paths extending along the width over the entire length or at any partial sections thereof, and be interrupted along the length by capacitances and feed line connections.
The windings of the coils may be connected in series or in parallel. With multi-winding coil configurations, one distinguishes between parallel-parallel coil configurations, parallel-series coil configurations, series-series coil configurations and series-parallel coil configurations. The first term characterizes the wiring of the windings within a coil, and the second term characterizes the wiring of the two coils of the coil configuration.
Series-series coil configurations have previously mainly been used in NMR (see e.g. U.S. Pat. No. 4,398,149). These have a relatively high inductance, such that the lossy inductances of the feed lines and the additional inductances which are used in the tuning circuit of multi-frequency probe heads can be neglected in the overall coil configuration. This renders the coils highly efficient. However, the eigenresonances of the multi-winding series-series coil configurations are too low for high field NMR (static field B0>7T) due to the high inductances. This produces poor RF field homogeneities and makes tuning of the measuring frequencies for e.g. 13C and 31P difficult. This problem can be solved by the reduced inductance of multi-winding series-parallel coils (see e.g. U.S. Pat. No. 6,175,237). Very large measuring samples that are used e.g. in MRI (magnetic resonance imaging) may require use of single-winding coils or parallel-parallel coils (see e.g. U.S. Pat. No. 6,060,882). In this case, the inductances of the feed lines and the circuit may, however, increase considerably faster than those of the coil configuration.
In addition to Helmholtz and saddle coils, further coil types are used in NMR such as e.g. solenoid, butterfly and meanderline coils and planar coils of the most different shapes.
Alternatively, resonator configurations may be used in the probe head which may consist of one or more resonators. So-called Birdcages, Alderman-Grant resonators or transmission line (slotted tube) resonators and also planar, self-resonant structures such as e.g. spiral resonators are frequently used in NMR.
Combinations of coil and resonator configurations are often used to build an NMR probe head for more than one operating frequency. This may be realized e.g. by positioning a first coil or resonator configuration concentrically in a second coil or resonator configuration, and aligning both such that the RF magnetic fields produced thereby are largely orthogonal to each other.
One important aim of all coil and resonator configurations is to minimize coupling between the coil/resonator configurations. Unless otherwise explicitly stated, the coil configuration and the resonator configuration are treated equally below, since the observations are equivalent for both.
A coupling between the at least two coil configurations is undesirable for many reasons:                1. In case of resonance, current flows from one coil configuration to the other through coupling. The currents of the “inner” coil configuration are thereby removed from the measuring sample, the efficiency of the configuration is reduced and thereby also the sensitivity of the probe head at that/those measuring frequency/frequencies that resonate in the inner coil configuration.        2. If a coil configuration has a considerably higher Q value than another one, residual coupling produces strong attenuation of the coil configuration with the higher Q value. This occurs, in particular, when e.g. superconducting materials are used in one coil configuration, with the other using normal conductors.        3. When the coupling between the coil configurations can be neglected, each coil configuration can be separately tuned to the at least one measuring frequency. This simplifies construction of the probe head as well as the tuning process when the sample is changed during operation.        4. When the coupling can be neglected, no cross talk can be measured between the measuring channels. This facilitates e.g. decoupling experiments.        
In particular, for conventional multi-winding series-parallel coil systems as described in U.S. Pat. No. 6,175,237 and the references cited therein, coupling of two coil systems (coil configurations) can be minimized, but not be eliminated.
It is therefore the underlying purpose of the invention to provide an NMR probe head, wherein coupling between two coil/resonator configurations is further reduced.